fix no.1:
make sure you have the ‘ca.crt’ in a folder in /etc/docker/certs.d that matches your registry tag exactly (note: you need to do this ‘ca.crt’ on EVERY host that needs to talk to the registry)

e.g.
if your command is docker tag 7b9b13f7b9c0 dockeregistry:5000/ubuntu
then you have to have a ‘ca.crt’ in /etc/docker/certs.d/dockeregistry:5000 (includes colon and port number)

fix no.2:
if fix no.1 doesn’t work and you’re still getting the ‘unknown authority’ error try the following from the docker product manual for your distribution.